In a laser beam scanning system, it is advantageous to combine multiple laser beams, each of differing wavelengths, to achieve a multi-color scan exposure of a medium, or to increase throughput by writing plural lines simultaneously, or to provide output beams of selectable wavelengths (such as the provision of a read beam at one wavelength and a write beam at another wavelength). One conventional approach teaches the superimposition or combination of plural input beams into an output beam by use of a beam splitter operated in a reversed orientation.
However, the beam splitters proposed heretofore for beam combination, such as multi-layer dielectric coated mirrors, grating element, and pellicle beam splitters, exhibit a host of drawbacks, The named beam combiners are inefficient (i.e., they exhibit an undesireable amount of beam power loss) and are undesireably sensitive to factors such as the differential in the input beam wavelengths, the separation angle of the input beams, and the ambient temperature. These drawbacks are especially disadvantageous when the beam combiner is operated in a scanner wherein the radiometric accuracy of the output beam must be accurately maintained.
For example, as proposed by Tateoka in U.S. Pat. No. 4,634,232, two glass triangular prisms, one of which is coated with a polarizing multilayer mirror coating, are cemented together so that the mirror surface is on the cube diagonal. The multilayer mirror is typically a series of quarter wave interference coatings. The mirror surface transmits a first beam of one polarization and reflects another beam polarized orthogonally to the first beam. The passband, and hence the reflectivity and transmissivity of the mirror, is temperature, wavelength, and angle sensitive. Typically, only 80% of P-polarized light is transmitted and 90% of S-polarized light is reflected. Furthermore, the polarization purity of each beam component in the resulting output beam is unacceptable for some applications. The leakage component causes interference and the resultant intensity fluctuations can be quite objectionable.
Beam combiners formed from certain birefringent elements have been proposed for single wavelength multiple beam combiners. For example, a beam combiner in the form of a Wollaston or Rochon prism was proposed by Tatsuno et al. in U.S. Pat. No. 4,822,151 to combine phase-locked input beams emanating from a diode laser array operating at a single wavelength. However, such a beam combiner would suffer from undesireable angular variation in the output beam if the output beam is formed from input beams of differing wavelengths.